Argues that there has been a change of norm in relation to the legitimacy of humanitarian intervention in the 1990s. It shows how humanitarian justifications for the use of force lacked legitimacy in Cold War international society, focusing on the cases of India, Vietnam, and Tanzania's interventions in the 1970s. This reflected the dominance of pluralist international society thinking in shaping the legal rules and institutions of international society. By focusing on cases of intervention in Iraq, Somalia, Rwanda, Bosnia, and Kosovo, the second part of the book shows how a new solidarist conception of international society shaped Western interventions in the 1990s. In arguing that a new norm has developed that has facilitated new state actions; the book identifies two key limits to this norm: first, military intervention justified on humanitarian grounds requires UN Security Council authorization; second, whilst new norms enable new actions, they do not determine that intervention will take place when it is urgently needed as in Rwanda.